Proper regulation of blood flow is essential for the health and function of the retina. Flicker stimulation evokes increases in blood flow in the retina a response termed functional hyperemia. This response brings additional oxygen and glucose to active neurons. We have recently demonstrated that blood flow is differentially regulated within the three capillary layers in the retina. Flickering light evokes far greater increases in capillar diameter and blood flow in the intermediate capillary layer than in the deep and superficial layers. The goal of this project is to determine the mechanisms responsible for this differential control of blood flow and to establish whether this control is altered in diabetic retinopathy, where functional hyperemia is known to be disrupted. The aims of the project are: Aim 1. Test the hypothesis that selective dilation of intermediate layer capillaries is driven by the activity f nearby neurons. The large flicker-evoked responses of intermediate layer capillaries may arise because synaptic activity near these capillaries is greater than activity near the other capillary layers, leading to greater release of vasodilating agents. This hypothesis will be tested by using local Ca2+ signals within Mller glial cells as an indicator of nearby synaptic activity. Calcium signals will be imaged in different retinal layers using transgenic mice expressing the genetically encoded Ca2+ indicator GCaMP3 in Mller cells. Aim 2. Test the hypothesis that flicker-evoked dilations of capillaries is mediated by production of arachidonic acid metabolites. Flicker-evoked capillary dilations will be monitored in the rat in vivo in the three capillary layers as the signaing of candidate vasodilators is blocked by intravitreal injection of synthesis inhibitors. Candidate vasodilators will also be injected into the vitreous humor and the resulting changes in capillary diameter in the three capillary layers will be monitored. Aim 3. Test the hypothesis that dilation of intermediate layer capillaries is compromised in diabetic retinopathy. Capillary diameter in the three capillary layers will be measured at 0.5, 1, 3 and 6 months after diabetes induction by streptozotocin injection. The effect of aminoguanidine, which reverses the loss of arteriole dilation in the diabetic retina, on capillary dilation will be investigated.